This invention relates in general to electric motors and in particular to an improved structure for retaining a plurality of bobbin wound coil assemblies within such an electric motor. One specific application for this invention is in a variable reluctance electric motor, although the invention may be used in other electric motor structures as well.
Electric motors are well known devices which convert electrical energy to rotary mechanical energy. To accomplish this, electric motors establish and control electromagnetic fields so as to cause the desired rotary mechanical motion. There are many different types of electric motors, each utilizing different means for establishing and controlling these electromagnetic fields. Consequently, the operating characteristics of electric motors vary from type to type, and certain types of electric motors are better suited for performing certain tasks than others.
Synchronous motors constitute one principal class of electric motors. The two basic components of a synchronous motor are (1) a stationary member which generates a rotating electromagnetic field, generally referred to as the stator, and (2) a rotatable member driven by the rotating magnetic field, generally referred to as the rotor. Synchronous motors are characterized in that the rotational speed of the rotor is directly related to the frequency of the electrical input signal applied thereto and, therefore, the rotational speed of the electromagnetic field generated thereby. Thus, so long as the frequency of the applied electrical input signal is constant, the rotor will be driven at a constant rotational speed. Within this broad definition, however, the structure and operation of synchronous electric motors vary widely.
One variety of synchronous electric motor is known as a variable reluctance motor. Variable reluctance motors operate on the principle that a magnetic field which is created about a component formed from a magnetically permeable material will exert a mechanical force on that component. This mechanical force will urge the component to become aligned with the magnetic flux (lines of force) generated by the magnetic field. Thus, by using the stator to establish and rotate a magnetic field about a rotor formed from a magnetically permeable material, the rotor can be driven to rotate relative to the stator. The resistance to the passage of this magnetic flux from the stator to the rotor is referred to as reluctance. The magnitude of this reluctance changes with the rotational position of the rotor relative to the stator. Thus, electric motors of this type are commonly referred to as variable reluctance motors.
In a basic variable reluctance motor structure, this operation can be accomplished by providing a generally hollow cylindrical stator having a plurality of radially inwardly extending poles formed thereon. A coil or winding of an electrically conductive wire is provided about each of the stator poles. Concentrically within the stator, a cylindrical rotor is rotatably supported. The rotor is provided with a plurality of radially outwardly extending poles. However, no electrical conductor coils are provided on the rotor poles. By passing pulses of electrical current through each of the stator coils in a sequential manner, the stator poles can be selectively magnetized so as to attract the rotor poles thereto. Consequently, the rotor will rotate relative to the stator.
To optimize the operation of the variable reluctance motor, the magnitude of the electrical current which is sequentially passed through the stator coils is typically varied as a function of the rotational displacement of the rotor, as opposed to simply being supplied in an on-off manner. For example, the magnitude of the electrical current passed through a particular stator coil can initially be large, but decrease as the rotor pole rotates toward it. Consequently, the stator coil is prevented from continuing to attract the rotor pole toward it when the rotor pole has rotated to a position near or adjacent to the stator pole. This facilitates the rotation of the rotor at a more uniform speed.
As mentioned above, coils of electrically conductive wire are provided about each of the stator poles in a typical variable reluctance motor. In the past, these coils have been formed by winding the electrically conductive wire (which is coated with an electrically non-conductive insulator) about the poles of the stator. The wire used in such coils has typically been conventional wire having a circular cross sectional shape. While such wire is commonly available and relatively inexpensive, the circular cross sectional shape of the wire does not lend itself to high density winding. Also, a special winding machine must be provided to wind the wire about the poles of the stator within the electric motor.
More recently, it has been found desirable to form the wire used in such coils from insulated foil wire. Foil wire lends itself to higher density windings. Also, a special winding machine is not necessary because the foil wire can be wound on a mandrel outside of the electric motor, then installed therein. While electromagnetic coils formed from both round and foil wire have been effective, it would be desirable to provide an improved structure for an electric motor in which the coils are wound upon bobbins, and wherein an improved structure is provided for retaining these coils on the poles of an electric motor.